Exogenous arachidonic acid (AA) incubated in presence of male rat hypothalamus, shows a low rate of conversion (<1%) of the substrate with a major product, identified as 12-hydroxyeicosatetraenoic acid (12-HETE) by reverse phase-high performance liquid chromatography (rpHPLC) and gas chromatography-mass spectrometry (GC-MS). Furthermore, immunoreactive 12-HETE estimated after purification on rpHPLC is produced by hypothalamus slices or median eminences (MEs) incubated in absence of any exogenous precursor. The effect of 12-HETE was tested on the release of LHRH from rat MEs after a 30-min incubation and was compared to the effect of another lipoxygenase product, 5-HETE, and to the well-known stimulatory effect of prostaglandin E<sub>2</sub> (PGE2). The three AA metabolites stimulate LHRH release. A significant stimulatory effect on LHRH release is obtained with 10<sup>–9</sup> M of 12-HETE and only with 10<sup>–8</sup> M of 5-HETE or PGE2. Furthermore, the effect of higher concentrations is different according to the eicosanoid tested. The maximal response (176% of the control) is reached with 12-HETE at 10-<sup>8</sup> M No significant change is observed at 10<sup>–7</sup> and 10<sup>–6</sup> M The response with 5-HETE is also maximal (162% of the control) at 10<sup>–8</sup> M but decreases significantly (only 117% of the control) at 10"<sup>6</sup> M The amplitude of the response to PGE2 is larger and higher, reaching a plateau (300% of the control) at 10<sup>–6</sup> M 12-HETE has no effect on somatostatin (SRIF) release, as already known for PGE2. These results are the first evidence of selective effect of lipoxygenase products on the release of neuropeptides from rat hypothalamus. The identification of 12-HETE as endogenous metabolite is in favor of a physiological role of the compound.